The present disclosure relates generally to protection against residual currents. In particular, the present disclosure relates to a residual current protective device for an electric circuit with at least one electrical consumer, an electric circuit with at least one such residual current protective device, an aircraft element or an aircraft assembly comprising at least one such residual current protective device and/or at least one such electric circuit, an aircraft comprising at least one such residual current protective device and/or at least one such electric circuit, the use of the residual current protective device on board an aircraft and a method for residual current protection.
In electric circuits with electrical consumers, residual currents can occur for various reasons, which currents flow out of the electric circuit to earth (ground). For example, hazards may arise for persons when touching live parts of electrical systems. Furthermore, residual currents can ignite insulation, even as very small currents. Machinery can also be damaged by a leakage short occurring, for example. The aforementioned examples can be summarized under the terms personal protection, fire protection and machinery protection. To avoid such consequences, various protective devices have been used successfully for years.
An important protective device for the power grid is realized by the so-called residual current circuit breaker. Residual current circuit breakers, also commonly known as “RCCBs,” are the most frequently used devices from the higher-level group of “residual current devices” (RCD). Residual current devices of any type prevent dangerously high residual currents to earth and thus contribute significantly to reducing life-threatening electrical accidents in low-voltage networks.
Residual current protective devices protect against the continued existence—not the emergence—of an excessively high touch current. For personal protection they are an efficient means of avoiding dangerous electrical accidents, in particular earth leakages via the body. Such protective devices typically disconnect the circuit from an operating current (also termed tripping current) of 10 mA or 30 mA, for example. Another very substantial aspect for the use of residual current protective devices is fire protection. For technical fire safety reasons, protective devices, alarm or monitoring apparatus with a rated residual current/operating current of, e.g., 300 mA are used. The aforementioned machinery protection can also be achieved with the aid of residual current protective devices. Residual current protective devices with an operating current of 500 mA or more, for example, are used for machinery protection.
In addition to the aforementioned residual currents, so-called leakage currents (often also termed stray currents) can occur for various reasons in electric circuits with electrical consumers. A current can be described as a stray current if it basically flows normally to the earthing (grounding) or equipotential bonding system or to a protective earth conductor of an electrical installation via current paths not belonging to the operating circuit. The current paths not belonging to the operating circuit can be current paths that have been deliberately provided, for example, or undesirably occurring current paths. A capacitive filter at inputs and/or outputs of the connected electrical consumers (equipment), such as, e.g., EMC filters, can be cited as an example of a deliberately provided current path. In many spheres, such as, e.g., on board aircraft, electrical appliances are often installed with a very high density, so that high-frequency interferences can affect neighboring appliances in spite of the short distances between the appliances. Electrical fault currents can be discharged to the aircraft earth with the help of these filters. Examples that can be cited as undesirably occurring current paths are parasitic leakage capacitances of cables and lines or parasitic leakage capacitances in connected electrical equipment.
A residual current circuit breaker disconnects the monitored circuit from the rest of the power grid as soon as current flows on an unpermitted or inexplicable path. To do this, the residual current circuit breaker compares the level of the current flowing out with that of the returning current. In an intact system, the sign-dependent sum of all currents flowing through the residual current circuit breaker must be zero. Expressed in another way, the current to the consumer must be as great as the current flowing back from the consumer. Residual currents can occur, however, if a (residual) current flows through the human body or via defective insulation, for instance. As soon as a partial current is discharged to earth anywhere in the circuit, the sum of the outward and returning current in the summation current transformer of the residual current circuit breaker is no longer zero. A current difference occurs, which leads to tripping of the residual current circuit breaker and thus to the complete disconnection of the circuit. The comparison takes place in a summation current transformer. This has two or more cable cores (primary windings) running through it. They are routed so that their induction effect is normally mutually cancelled out, no magnetic flux is induced in the transformer core and no secondary current flows. The transformer thus “adds” all currents flowing to and from the consumer with sign. If a partial current flows to earth from one core (residual current), then the sum of outward and returning currents in the transformer is no longer zero. This results in a current in the secondary winding (trip coil). The secondary current triggers a relay (switching lock), which disconnects all poles of the cable.
This means that if a certain residual current is exceeded, usually 30 mA in household systems and public buildings, known residual current circuit breakers disconnect all poles of the monitored circuit, meaning all conductors up to the protective earth conductor, from the rest of the grid. If the returning current is reduced solely on account of a leakage current (e.g., due to defective insulation or to a capacitive filter arranged in the circuit) by 30 mA, for example, the circuit breaker trips, although no hazardous situation might exist.
It is true that conventional residual current protective devices guarantee reliable tripping in the case of a hazardous residual current. On the other hand, as outlined above by way of example, it can occur that the protective devices trip although no hazardous residual current exists. Excessively frequent, unnecessary disconnection of the circuit can lead to a reduction in the operational availability of electrical systems.
It is desirable, therefore, to provide a technology that trips in the event of dangerously high residual currents, but reduces the unnecessary disconnection of electrical consumers.